Lab Organic Test
What is the purpose of melting point determination
Melting points tell you the purity of substance, the higher the melting point range is a good indicator of purity
(a) Is it possible that the synthetic form of tartaric acid is the meso isomer? Why or why not?
No, because they have different melting points.
Which IR bonds are not typically IR active?
Non-polar bonds
Which IR bonds are typically IR active?
Polar bonds
Which bonds absorb strongly on IR?
Polar bonds
● What happens if you do not vent the sep funnel during extractions?
Pressure will build up and push the stopper out and you lose all your compound
● How is percent recovery calculated?
% recovery: (g of pure material /g of impure material) * 100%
A student wants to recrystallize compound A using ethanol as a solvent. The boiling point of ethanol is 78C. The solubility of compound A in ethanol is 0.8 g per 100 mL at 0C and 5.0 g per 100 mL at 78C. Using this information, answer the following questions. Clearly show your work for full credit.
(0.8 g sample)/(100 ml)= ((X)g )/(240 ml) (X)=((0.8 g x 240 ml))/(240 ml)=1.92 g
● How were melting points used in recrystallization?
After the crystals formed, they were melted to see the melting point and determine which solution we had. The melting point range determined the solution.
What does IR measure?
-molecular vibrations in bond stretching/bending by passing IR light and measuring absorption, plotted as % transmittance -there must be a change in dipole moment* *symmetrical molecules will NOT have these characteristics and will NOT record an absorption
2. Why is pencil used instead of pen to mark the origin of the TLC plate?
A pencil is used because the lead does not dissolve into the solvent unlike the ink of a pen. In addition, the ink could possibly move up the TLC plates.
● How is a solvent chosen for recrystallization?
A solvent is chosen by being a poor solvent at room temperature and a great solvent at high temperature.
What does conjugation mean?
Alternates double bond, single bond, double bond, single bond
1. Which would be more reactive when treated with silver nitrate in ethanol, benzyl chloride or bromobenzene? Explain.
Aryl halides (bromobenzene) are unreactive to nucleophilic substitution.
Why does conjugation affect IR frequency
Changes the electronic structure which changes absorption
How can extraction be used to separate acidic, basic, and neutral components?
Converting acids and bases to their corresponding salts
3. Describe the process you would follow to separate a mixture containing benzoic acid and benzophenone. Hint: consult Table 1 from your procedure for information on these two compounds.
Dissolve both in ether and extract with NaHCO3.
What does IR tell you about the compound?
Gives you the functional groups and the peak tells you if there is either C-H, C=C, C-O bonds, also cis-trans isomerism, alkene compounds, and intermolecular bonding
What is IR?
IR is determines structure , the amount of light absorbed by the sample is varied.
A student wants to recrystallize compound A using ethanol as a solvent. The boiling point of ethanol is 78C. The solubility of compound A in ethanol is 0.8 g per 100 mL at 0C and 5.0 g per 100 mL at 78C. Using this information, answer the following questions. Clearly show your work for full credit. a. What is the minimum amount of ethanol needed to recrystallize 12.0 g of compound A?
If 5 g needs 100 ml then (12×100) ÷5= 240 ml of ethanol.
● How can a known compound be used to determine the identity of an unknown compound using melting point data?
If the melting point of the unknown is in the range of the known compound, then you can identify it.
d. Forgetting to remove the TLC plate when the solvent has reached the top of the plate
If the plate is not removed when the solvent is at the top, then the sample will travel too far and the Rf value will be inaccurate.
● Is it better to make one large extraction or several smaller extractions? Why?
Several small extractions because you retain more product with more extractions, you get a better percent yield.
● What determines which layer is on top in an extraction?
The density, the solvent with the lowest density will be the top layer.
Unknown X melts at 85.0-86.0C. Compound Y also melts at 85.0-86.0C. What results would be observed for the melting point of a mixture of X and Y if: (a) X and Y are the same compound?
The mixture MP will be the same as the individual MP
● How can you determine which layer is the aqueous layer and which is the organic layer?
The organic layer will have the lowest density will be the top layer, the organic layer will ALMOST always be lighter than the aqueous layer
Unknown X melts at 85.0-86.0C. Compound Y also melts at 85.0-86.0C. What results would be observed for the melting point of a mixture of X and Y if: X and Y are not the same compound?
The range for the mixture will be lower
● What if the mp range of a compound is short, but not very close to the expected mp temperature?
The sample is not quite 100% pure
1. What will be the result of the following errors in TLC technique? a. Too much sample applied
The sample will streak
1. Suppose you wanted to separate components of a mixture using column chromatography. You ran TLC on your mixture and saw a good separation of components using methylene chloride as the developing solvent. But when you ran the column, you inadvertently eluted the column with methanol. What would happen?
The separation of spots would be different The methanol is polar than the methylene chloride. Methanol is miscible with water and cannot form the layers need for separation.
What is the significance of a broad mp range?
The substance is not as pure
● If the melting point range is short (2 degrees or less) what significance does this have?
The substance is purer, the sharper the range the better
2. What would be the effect on the results of a column chromatography if: (a) solvent was not maintained above the stationary phase?
The would be no separation or absorption of the material.
2. Determine the configuration at each stereocenter of (-)--pinene. You may find it helpful to make a molecular model to visualize the three-dimensional structure of (-)--pinene.
There are two stereocenters in (-)--pinene, both resulting in S-configurations.
(b) Why is the meso isomer optically inactive?
They do not cancel out. It is achiral so has no optical activity.
What is the purpose of recrystallizing a product or isolated material?
To purify a solid compound
Compound B is quite soluble in toluene, but only slightly soluble in petroleum ether. How could these two solvents be used in combination to recrystallize compound B?
To recrystallize compound B (which is soluble in toluene) both solvents would have to be used to form pure crystals. Since the compound is insoluble in petroleum ether, it should be heated in ether. After heating, add a small amount of toluene (like the charcoal) until it also dissolves in the solution. Once the solution cools, purer crystals should be produced.
● What is the purpose of boiling stones?
To stop excess bumping when time to heat the solution
What effect would each of the following have on the observed melting-point range of a sample?
Uneven heating leads to a wider melting range
● What solvent was used for recrystallization of acetanilide?
WATER
Can IR be used to identify a structure?
YES
1. Assume that during an extraction you have lost track of which layer is the aqueous layer and which is the organic layer. How could you quickly and easily determine which layer is which?
You can determine the layer by adding a small amount of water to aqueous layer and organic layer to see which layer the water adds to.
(b) you did not number the test tubes in which you are collecting fractions?
a. It will make it harder to know which sample truly contained the caffeine and thus harder to decide what sample to combine to create the final sample, and thwarting the melting point of the sample.
● What happens during recrystallization?
chemist use different solubilities of compounds in different solvents as well, dissolving the material to be crystallized in a hot solvent and cooling the solution slowly, the solubility of a dissolved material decreases with decreasing temperatures, therefore, the solution will become oversaturated and the solid will separate from solution as it is cooled. If the two compounds dissolved in the solvent have differing solubilities, it becomes possible to separate these two compounds.
● What is a back-extraction?
doing an extraction again... for example using ether and water in an extraction you put the water back into the funnel to do another extraction with fresh ether.
(c) If the synthetic form is not the optically inactive meso isomer, why might the synthetic form have an optical rotation of 0°?
its a racemic mixture
How conjugation affect IR frequency
lowers the frequency
1. What will be the result of the following errors in TLC technique? a. solvent is too polar
no separation or spots
Which bonds do not absorb strongly on IR?
non-polar
How can you tell if the extracted solution is "dry"?
the solution is milky white
● What is an extraction?
the technique used for selectively separating a compound from a mixture, A mixture of two compounds, one water soluble and one water insoluble can be separated by adding both water and organic solvent, if the organic solvent and the water are immiscible the two layers can be separated thereby separating the two compounds
● What is the purpose of using a drying agent on the organic layer after a separation?
to create a solid
Why do we use boiling stones?
to prevent over boiling of solution
● What is activated carbon used for?
to remove color impurity
baseline of IR spectroscopy
top of the IR
● If a solid is not dry, what effect does this have on the melting point determination?
water will depress the melting point and give wide range melting point.